Powerline is only for your internal network, it has nothing to do with your ISP. Basically it goes ISP (Comcast, Time Warner, Cox, etc) goes in to your Modem like normal, your modem goes in to your router (assuming your modem doesn't have a router built in), your router goes out to the rest of your network. You plug a ethernet cable from your router in to you powerline adapter and your powerline adapter plugs in to an electrical socket socket. Then you plug another powerline adapter in to the electrical socket in the room of your choosing and that creates the network through your existing electrical wiring in the house. Now since your wiring is older, your performance may be anywhere from good to horrible, its a complete gamble and only using it will tell you how it'll perform. Different electrical sockets in the same room will give you different performance, so it takes testing it to find the best place to plug in. When it works its fantastic, I use a set of them myself. Since its all dependent on your wiring, the best thing you can do is make sure to order from someplace with a good return policy (amazon is my preferred) and try it out.
For a basic "what is powerline" check out this page.
http://www.pcadvisor.co.uk/feature/network-wifi/what-is-powerline-3491484/
The adapters mentioned in it are a bit out of date and may be UK specific. I use a set of ZyXEL PLA5405KIT with mine and have no issues with speed. The Netgear Powerline 1200 PLP1200-100PAS (with or without the extra outlet) kit has also been pretty well reviewed.